Editor’s Note, 6 May 2015: For new results of this investigation, which tie slow fault slip to earthquakes strong enough to produce tsunami, read the Eos.org news story.

Two recent studies and an upcoming International Ocean Discovery Program (IODP) drilling project target a better understanding of the causes and consequences of shallow slow slip events (SSEs). These events, where large portions of New Zealand’s North Island move eastward at a speed that is too slow to detect using seismometers, occur along the Hikurangi margin, which runs along the eastern coast of the North Island.

Studies are already under way to help understand the spectrum of fault slip on subduction thrust faults, which generate Earth’s most powerful earthquakes.The uplifted Hikurangi margin provides an important location for amphibious investigations of the plate boundary and is the focus of several large-scale international investigations into shallow slow slip and the seismogenic zone. The U.S. National Science Foundation (NSF) Geodynamic Processes at Rifting and Subducting Margins (GeoPRISMS) program has selected this region as a focus site for its Subduction Cycles and Deformation initiative. As GeoPRISMS activity begins ramping up at this site over the next several years, studies are already under way to help understand the spectrum of fault slip on subduction thrust faults, which generate Earth’s most powerful earthquakes.

Slow Slip Events

During SSEs, millimeters to decimeters of aseismic fault slip continue over days to months at rates between long-term plate motion and seismic slip. Although many of these events are found at depths of 25 to 50 kilometers, newly imaged shallow SSEs (less than 15 kilometers deep) provide a unique opportunity to investigate this phenomenon nearer to the source of slow slip [Saffer and Wallace, 2015].

Subduction thrusts host a spectrum of fault slip behaviors that range from episodic aseismic slip events, where the movement produces no earthquake activity, to the largest and most destructive earthquakes and tsunamis on Earth. SSEs often occur along portions of the plate interface where slip behavior transitions from aseismic slip to deep interseismic coupling (strain accumulation between earthquakes) and stick slip (a locking of the plate boundary between earthquakes and release of accumulated stress in earthquakes). These shallow SSEs may hold clues that increase our understanding of fault slip behavior, including earthquakes [e.g., Schwartz and Rokosky, 2007].

The Hikurangi Trough

Fig. 1. Location map of the Hikurangi Ocean Bottom Investigation of Tremor and Slow Slip (HOBITSS) and Subduction Thrust Investigation of New Zealand using Geothermics and Seismics (STINGS) experiments and the planned International Ocean Discovery Program (IODP) drilling transect. The map shows the locations of absolute pressure gauges (APG), ocean bottom seismometers (OBS), and ocean bottom electromagnetic instruments (OBEM). Orange contours show slippage distances for the 2010 slow slip event [Wallace and Beavan, 2010]. The maximum slip of 120 millimeters occurred within the innermost contour line. The contour interval is 20 millimeters, and the outermost contour represents 20-millimeter slip. The inset shows the experiment location: New Zealand (NZ); Australia (Aus); and the Hikurangi (HT), Kermadec (KT), and Tonga Trench (TT) systems.The Hikurangi Trough (Figure 1) lies at the southern end of the Tonga-Kermadec subduction zone. It is the site of westward subduction of the thick and buoyant Hikurangi Plateau, an early Cretaceous (120 million years ago) large igneous province [Davy et al., 2008]. The buoyancy of the Hikurangi Plateau has uplifted the fore arc, the region between the oceanic trench and its associated volcanic arc, so that the subduction thrust is located only 12 kilometers beneath the coastline.

At the northern Hikurangi subduction margin, well-documented SSEs occur at depths less than 15 kilometers approximately every 18 months. These events last for approximately 1 to 2 weeks and produce horizontal displacements of as much as 3 centimeters at onshore continuous GPS sites [Wallace and Beavan, 2010]. These SSEs are associated with tremors [Kim et al., 2011] and elevated levels of microseismicity [Delahaye et al., 2008]. In addition to the SSEs, this part of the subduction interface has generated tsunami earthquakes with a surface wave magnitude of 7.0–7.2 that originated near the trench in March and May 1947 [Doser and Webb, 2003].

Project Design and Technology

The Hikurangi Ocean Bottom Investigation of Tremor and Slow Slip (HOBITSS) offshore seismic and geodetic experiment is jointly funded by the NSF, Japan, and New Zealand. This project is exploring the relationship between slow slip events, tectonic tremor, and seismicity along the northern Hikurangi Margin (Figure 1).

Five short-period and 10 broadband period ocean bottom seismometers, 24 absolute pressure gauges (APGs), and 3 ocean bottom electromagnetic instruments were deployed in May 2014 using the New Zealand R/V Tangaroa. The U.S. R/V Roger Revelle successfully recovered all instruments in June 2015. We are using the data to investigate the offshore occurrence and distribution of slow slip and the spatial and temporal relationship between slow slip and seismicity on and near the megathrust.

One of the most exciting and novel aspects of HOBITSS is the use of a dense network of APGs (<10 kilometer spacing) to detect vertical deformation of the seafloor during large slow slip events that occurred beneath the array between late September and early October 2014. These data could reveal the detailed distribution of slow slip on the shallow megathrust for the first time.

A second research cruise, the Subduction Thrust Investigation of New Zealand using Geothermics and Seismics (STINGS), aboard the R/V Revelle during May and June 2015, is the first part of a field and modeling program to investigate the temperature and seismic structures of the Hikurangi margin and their relationship to slow slip events and interseismic coupling of the subduction thrust (Figure 2).

Heat flow measurements, colocated with previously collected seismic reflection lines, indicate that the background heat flow is consistent with simple models of conductive cooling in the oceanic lithosphere. In combination with global compilations of heat flow observations, such models have often led to the expectation that thermally significant fluid flow might not occur in oceanic crust older than 65 million years old. However, despite the 120-million-year age of the Hikurangi Plateau, preliminary interpretation of the heat flow measurements suggests that heat transport by fluid flow is locally important in the STINGS study area.

Future Goals

Both of these experiments are providing important context to guide planned drilling aimed at understanding the origins of slow slip. The Facilities Board for the U.S. drilling vessel JOIDES Resolution has committed to drilling a transect of boreholes in 2018 as a part of this project.

North Hikurangi offers an opportunity for unprecedented high-resolution imaging, direct sampling of rocks, and measurement of in situ properties that is not possible in deeper slow slip event regions.These boreholes will address a suite of scientific objectives. One main objective is characterizing the in situ state and composition of the incoming plate and shallow plate boundary fault near the trench. Another is installing borehole observatory instruments to measure temporal variations in deformation, hydrogeological, geochemical, and thermal conditions along a transect of holes above the SSE source.

The close proximity of SSEs to the Earth’s surface at north Hikurangi offers an outstanding opportunity for unprecedented high-resolution imaging of the SSE source and direct sampling of rocks and measurement of in situ properties in the slow slip event source area that is not possible in deeper SSE regions. International initiatives such as HOBITSS and STINGS, as well as planned IODP drilling and three-dimensional seismic acquisition, will help to resolve many outstanding questions about why subduction thrusts slip in slow slip events and the relationship of SSEs to larger, damaging earthquakes.

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